Clutches are well known devices used to selectively connect a source of rotational power, such as the crankshaft of an internal combustion engine and its flywheel, to a driven mechanism, such as a transmission. When the engine is connected with the transmission by the clutch, vibrations are transmitted through the clutch and into the transmission and other drivetrain components, producing undesirable noise conditions such as gear rattle. Additionally, torque spikes of high magnitude at drivetrain resonances which over time could damage the transmission and/or the other drivetrain components.
Clutches have long employed a plurality of compression springs between a clutch hub engaged with a transmission input shaft and a clutch disc engaged with the engine flywheel. These springs are typically disposed in spring pockets circumferentially located around the clutch hub. Compression of the springs is limited by a stop disposed between the hub and the disc limiting relative rotation therebetween. The springs provide some isolation between the engine and transmission of firing pulses of the engine and other engine speed fluctuations. However, point loading between the springs and the spring pockets occurs at ends of the springs, producing wear of one or both parts. Also, within the range of travel permitted by the stop or stops, the springs tend to move relatively freely within the pockets, bowing and rubbing against the sides of the pockets. This spring motion and wear may potentially lead to the springs breaking, or to an increase in the size of the spring pocket, diminishing the dampening effectiveness of the clutch and potentially enabling the spring to escape the clutch driven disc assembly.
One path of vibration communication from the engine to the transmission particularly difficult to dampen while maintaining other desired clutch performance characteristics is the splined interface between the hub of the driven disc and the input shaft of the transmission. The engaging splines must fit loosely enough to slide into each other for assembly purposes, and therefore have no preload against each other. It is therefore necessary to have a small amount of lash between the mating splines. However, the engaging splines transmit engine vibrations into the transmission. Vibrations are similarly communicated through the splined interface in clutches employing twin driven discs. Each disc has a hub with a spline engaging an external spline of a common inner hub. The inner hub in turn has an inner spline engaging the transmission input shaft. To reduce the resultant potential for noise, it is known to provide spline interfaces with increased circumferential clearance between the splines, combined with an elastic spring or a frictional dampener between the splined elements. Known springs and frictional dampeners, however, have many parts and are difficult to assemble.
Also of concern is the ability of the driven disc assembly to become completely disengaged from the flywheel and the pressure plate which is influenced by the axial stiffness of the driven disc assembly. If the clutch driven disc hub axially hangs-up, or sticks, on the input shaft, then the clutch will not completely release, and the driven disc will tend to drag against the flywheel.
Yet another concern with twin driven disc clutches relates to their assembly. Although most driven discs are nearly symmetrical when viewed from the side, there is typically an axial offset in one direction or the other. Without careful attention by the assembler, one or both of the driven discs may be placed on the inner hub in the incorrect direction, an error which, even if caught immediately, will require additional time to correct.